Gas separation by pressure swing adsorption is achieved by synchronized pressure cycling and flow reversals over an adsorber that preferentially adsorbs a more readily adsorbed component relative to a less readily adsorbed component of the feed gas mixture. The total pressure is elevated during intervals of flow in a first direction through the adsorber from a first end (feed end) to a second end of the adsorber (product end), and is reduced during intervals of flow in the reverse direction. As the cycle is repeated, the less readily adsorbed component is concentrated in the first direction, while the more readily adsorbed component is concentrated in the reverse direction.
A “light” product, depleted in at least one more readily adsorbed component and enriched in at least one less readily adsorbed component, is then delivered from the second end of the adsorber. A “heavy” product enriched in the more strongly adsorbed component is exhausted from the first end of the adsorber. The light product is usually the desired product to be purified, and the heavy product often a waste product, as in the important examples of oxygen separation over nitrogen-selective zeolite adsorbents and hydrogen purification. The heavy product may be a desired product, as in the example of nitrogen separation over nitrogen-selective zeolite adsorbents. Typically, a feed fluid is admitted to the first end of an adsorber and light product is delivered from the second end of the adsorber when the pressure in that adsorber is elevated to a higher working pressure. Heavy product is exhausted from the first end of the adsorber at a lower working pressure. In order to achieve a higher purity light product, a fraction of the light product or gas enriched in the less readily adsorbed component is recycled back to the adsorbers as “light reflux” gas after pressure letdown, e.g. to perform purge, pressure equalization or repressurization steps.
The primary function of the PSA process is to separate at least one preferentially adsorbed component such as nitrogen from at least one less readily adsorbed component such as oxygen, usually to concentrate the oxygen as the desired product from air as the feed mixture. The present invention is concerned with problems caused by other, even more preferentially adsorbed components in the process gases or in the surrounding atmosphere, such as ambient water vapor or another vapor contaminant, whose very strong and sometimes almost irreversible adsorption may deactivate or poison the adsorbent to degrade its capacity and selectivity for the primary separation function.
There are numerous commercial processes using the above adsorptive phenomena, with a multitude of pressure envelopes. In VSA processes, the adsorbent is at least partially regenerated at a sub-atmospheric pressure, while in many PSA processes, the adsorbent is regenerated at close to atmospheric pressure. Many processes also regenerate the adsorbent at substantially higher than atmospheric pressures. The different PSA processes are not differentiated herein unless explicitly stated otherwise. “PSA” means that the adsorption step is carried out at a pressure higher than the desorption or regeneration pressure.
Many process improvements have been made to this simple cycle design in order to reduce power consumption, improve product recovery and purity, and increase product flow rate. These have included multi-bed processes, single-column rapid pressure swing adsorption and, more recently, piston-driven, rapid pressure swing adsorption and rotationally valved PSA (rotary PSA). Cycle frequency with conventional valves and granular adsorbents cannot be greatly increased, so adsorbent inventory is large. The trend toward shorter cycle times is driven by the desire to design more compact processes with lower capital costs, lower power requirements and more compact and lighter equipment.
PSA processes and apparatuses using at least one multi-port, multi-fluid distribution valve, often with components relatively rotating, are defined herein as rotary PSA. These apparatuses require dynamic sealing surfaces, some of which define the boundaries of process gas system containment and sometimes the ambient surroundings. Because of the relative motion of the moving surfaces, a tight fluid seal is not practicable, and some mass flow of components in the surrounding ambient gas or other process gas into the light gas is possible, even if there are pressure gradients opposing these mass flows across the dynamic seals.
Most commercial adsorption processes currently employ fixed-bed adsorbents usually in the form of beads or pellets. Typically, these beads or pellets range in size from about 1.5 mm to 4 mm.
Parallel passage extrudate monoliths of zeolite adsorbent for PSA air separation devices are disclosed in U.S. Pat. No. 4,758,253 (Davidson et al). U.S. Pat. Nos. 4,968,329 and 5,082,473 (Keefer), and U.S. Pat. No. 6,231,644 (Jain et al., which is incorporated herein by reference) disclose spirally rolled adsorbent sheets of 1 mm or less thickness for use in a layered structure (either laminate or monolithic) for use in PSA devices to achieve higher frequency operation from the conventional 45 seconds cycle period, in the range of less than 0.5 seconds to about 5 seconds cycle period, while preserving a low pressure drop and low power consumption. High-surface-area, laminated adsorbers, having adsorbent supported in thin sheets separated by spacers to define flow channels between adjacent sheets, and with the adsorbers mounted in a rotor to provide the PSA process valve logic with only one moving part, facilitate a high frequency PSA cycle that can be performed in an extremely compact apparatus as disclosed by Keefer et al., U.S. Pat. No. 6,051,050, and Keefer et al.'s U.S. patent application No. 60/285,527, the disclosures of which are incorporated herein by reference.
As used herein, fast cycle PSA or high frequency PSA or high speed PSA refers to PSA processes and apparatuses that operate with less than about one-minute total cycle periods. Non-conventional PSA refers to either fast cycle PSA, rotary PSA or both.
One factor is the greater sensitivity of high performance adsorbents to contaminants. The use of low silica-to-alumina ratio zeolites (exchanged with cations such as calcium or lithium that provide high selectivity to nitrogen) for oxygen production may contribute to create a more sensitive material towards water deactivation since such zeolites tend to adsorb water more strongly on some of the active sites.
Traditional PSA and membrane separation units tend to use diverse methods of removing water, such as cooling followed by condensation, membrane separation or adsorption. Adsorption processes for water removal are very common in PSA or VSA processes where two main components have to be separated and there is a few percent of humidity in the gaseous mixture.
The patent literature has examples of complex processes to remove water. Toyama et al. in U.S. Pat. No. 3,594,984 (1971) disclose a system where water and carbon dioxide are removed in separate vessels and purified air is then fed to vessels packed with adsorbent that preferentially adsorbs nitrogen. Smith et al. in GB 2 042 365 (1980) and Armond et al. in U.S. Pat. No. 4,144,037 (1979) use a dual layer system in each PSA vessel. The first layer is a desiccant adsorbent. The second layer preferentially adsorbs the less desired component of the mixture. This concept of a dual layer is quite popular since it avoids the cost of extra vessels, valves and piping required in more complex approaches.
Another approach frequently used is to have a single layer of adsorbent (e.g. a zeolite). The first part of the zeolite bed acts as a desiccant bed for the rest of the zeolite bed. This concept has been used successfully in low frequency PSA in reasonably large scale plants (40 ton per day).
It also has been recognized that humidity leakage into the product end of the adsorbers may be minimized by careful design of valve stem or rotor sealing arrangements to isolate interior flow passages communicating with the product end of the adsorbers from the external environment. Thus, Keefer et al. in U.S. Pat. No. 6,063,161 disclose multiple seals on the rotor and shaft of a product-end rotary distributor valve, with the product gas delivered through a chamber between those seals. Similar product end sealing arrangements for rotary PSA devices also are shown by Keefer et al. in International Publication WO 99/28013. Monereau et al. (EP 1,095,689 A1) also have disclosed improved valve stem sealing arrangements to prevent humidity ingress into the product end of PSA adsorbers.
All methods for protecting zeolite against humidity described above work quite well within the life of the plant. If the methods are inadequate for protecting the adsorbent separation layer beyond the life of the plant, this is not important for the conventional cases.
Industrial PSA and VPSA systems operate at low cycle frequency (with cycle times for typical processes ranging from about 1 minute to 10 minutes) using large inventories of adsorbent. These industrial processes are relatively insensitive to minor contamination by contaminants because: (1) the relatively large dimension of the adsorbers across which contaminant diffusion may occur; (2) the slow rate of any deterioration dependant on the cumulative number of cycles experienced; (3) the relative insensitivity to deactivation of a small fraction of a large adsorbent inventory; and (4) the relatively tight fluid sealing of static seals keeping the process gas from external ambient or feed gas conditions. Hence, a conventional system may operate for many years with no noticeable degradation.
Non-conventional PSA systems have been developed with operating frequencies up to two orders of magnitude faster than conventional industrial PSA processes. Consequently the adsorbent inventory is smaller by approximately the same factor of up to two orders of magnitude, and the dimension of the adsorbers across which contaminant diffusion may take place is also reduced by a large factor.